Quantum cascade lasers (QCLs) differ from traditional semiconductor diode lasers in that they do not use a p-n junction for light emission. Multiple active regions are “cascaded” so that each injected electron can emit multiple photons and therefore enhance laser gain. Each active region includes a multi-layered semiconductor material structure. This multi-layered semiconductor material structure is designed to have an electronic band structure that gives the desired emission wavelength, and is manufactured with nanometer-level thickness control.
Most commercially available QCLs are of the “edge-emitting” variety. In these, linear ridges are created by etching vertically through the layered structure, which has been grown on a wafer substrate. These are cleaved so that the active region comprises a rectangular ridge of several mm length, which forms a waveguide. The laser radiation is amplified by passing back and forth along the axis of the ridge. Laser emission occurs on the end facets of these ridges.
A current topic of research is ring-cavity surface-emitting (RCSE) QCLs. Here, the etched ridges are in the form of circles. The circular ridge forms a waveguide, inside which the laser radiation is amplified by going around and around. Emission occurs perpendicular to the plane of the ring, along the ring's symmetry axis, and perpendicular to the surface of the substrate. Emission occurs through the top of the ring and/or via the bottom and through the bottom side of the substrate. A feature of RCSE-QCLs is that the spatial distribution of their emission is in the form of a ring, or concentric rings, typically with a node (absence of radiation) at the center of the ring. While RCSE-QCLs have certain advantages over edge emitting QCLs, the central node in their beam profile may be a disadvantage for applications.